1. Field of the Invention
The present invention relates to fiber amplified spontaneous emission (ASE) light sources.
2. Description of the Related Art
Fiber ASE light sources are well known in the art. ASE sources have been advantageously used to provide wideband (e.g., on the order of 10 to 30 nanometers), single spatial mode light beams for multiple applications. For example, ASE sources have been used to provide laser light as an input to a fiberoptic gyroscope. For a description of an exemplary superfluorescent fiber source, see an article entitled "Amplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibers" by Emmanuel Desurvire and J. R. Simpson, published by IEEE, in "Journal of Lightwave Technology," Vol. 7, No. 5, May 1989.
An ASE light source typically comprises a length of single-mode fiber, with a core doped with an ionic, trivalent rare-earth element. For example, neodymium (Nd.sup.3+) and erbium (Er.sup.3+) are rare-earth elements that may be used to dope the core of a single-mode fiber so that the core acts as a laser medium.
The fiber receives a pump input signal at one end. The pump signal is typically a laser signal having a specific wavelength .lambda..sub.p. The ions within the fiber core absorb the input laser radiation at wavelength .lambda..sub.p so that electrons in the outer shells of these ions are excited to a higher energy state of the ions. When a sufficient pump power is input into the end of the fiber, a population inversion is created (i.e., more electrons within the ions are in the excited state than are in the ground state), and a significant amount of fluorescence is caused along the length of the fiber. As is well known, the fluorescence (i.e., the emission of photons at a different wavelength .lambda..sub.s) is due to the spontaneous return of electrons from the excited state to the ground state so that a photon at a wavelength .lambda..sub.s is emitted during the transition from the excited state to the ground state. The light which is emitted at the wavelength .lambda..sub.s from the fiber is highly directional light, as in conventional laser light. However, one main characteristic of this emission which makes it different from that of a traditional laser (i.e., one which incorporates an optical resonator) is that the spectral content of the light emitted from the superfluorescent fiber source is generally very broad (between 10 and 30 nanometers). Thus, the optical signal output by the fiber will typically be at a wavelength .lambda..sub.s .+-.15 nanometers. This principle is well known in laser physics, and has been studied experimentally and theoretically in neodymium-doped and erbium-doped fibers, and in fibers doped with other rare-earths, for several years.
Light emitted from ASE fiber sources has multiple applications. For example, in one application, the output of the ASE source is fed into a fiberoptic gyroscope. For reasons that are well understood by those skilled in the art, the fiberoptic gyroscope should be operated with a broadband source which is highly stable. Of the several types of broadband sources known to exist, superfluorescent fiber sources, in particular, made with erbium-doped fiber, have thus far been the only optical sources which meet the stringent requirements for inertial navigation grade fiberoptic gyroscopes. The broad bandwidth of light produced by erbium-doped fiber sources, together with the low pump power requirements and excellent wavelength stability of erbium-doped fiber sources, are the primary reasons for use of such sources with fiberoptic gyroscopes.
In an erbium-doped fiber, the emission of a superfluorescent fiber source is bi-directional. That is, the light which is emitted by the return of electrons to the ground state in the erbium ions is typically emitted out of both ends of the fiber. As described in U.S. Pat. No. 5,185,749, to Kalman, et al., for erbium fibers of sufficient length, the light propagated in the backwards direction (i.e., in the direction opposite that in which the pump signal propagates), has a very high quantum efficiency. Thus, it is advantageous to implement erbium sources so that the light emitted from the ASE erbium-doped source is emitted from the pump input end of the fiber (i.e., in the backward propagation direction).
An ASE source is generally implemented in one of two configurations. In a first configuration, called a single-pass ASE source, the superfluorescent source output power is emitted in two directions, one of which is not used. In the second configuration, called a double-pass ASE source, a reflector is placed at one end of the fiber to reflect the superfluorescent source signal so that the superfluorescent signal is sent a second time through the fiber. Since the fiber exhibits gain at the signal wavelength, the signal is amplified. One advantage of the double-pass configuration is that it produces a stronger signal. A double-pass ASE source configuration also produces output only at one port (i.e., in one direction). A disadvantage with such a configuration is that the feedback must be kept very low in order to prevent lasing (e.g., with use of an optical isolator).
For fiberoptic gyroscope applications, one critical measure of source performance is the stability of the source mean wavelength (for example, see U.S. Pat. No. 5,355,216 to Kim, et al.). As is well known in the art, stability of the source mean wavelength leads directly to the stability of the sensor scale factor error. The scale factor error is critical in determining an accurate measurement of the rotation of the gyroscope to precise values. Presently, sources exist which have a mean wavelength stability down to a few parts per million, assuming reasonable stabilization of system parameters such as pump wavelength, pump power, temperature, feedback, etc. However, a stability of less than one part per million in mean wavelength is desirable for some applications.